My LBS did suggest in the warm months adding 1 oz of Stans per month; I stretched it to every too much just due to laziness. In the summer I lost less than 5 psi/day (85/90 front/rear pressure). This winter I finding a negligible pressure drop. At my 195 lbs this is a lower pressure than I was running with tubes and the ride is appreciably better. Tires are Hutchinson 28mm Sector (or Secteur).
I suppose once or twice/year one ought to dump the old stuff out and inspect the inside of the tire. That's something I'll have to look into.
I'm running a regular Velocity A23 rim with two layers of Stans rim tape.
Just to circle back to my incident with the wire: I rode the bike with that embedded wire and it subsequently lost no pressure over night. If I had been doing a descent with a tube/tire combination a wire like that in the front tire could have been disastrous. I haven't experience a front flat on a descent and didn't switch to tubeless tires to mitigate this sort of risk, but every little bit helps.